is open. It reaches
the left-hand end of the tube, and we must imagine that when it gets
there it, in the same manner and by the proper connections, closes D.,
opens C., closes A. and opens B. If these mechanical movements are
completed it must be plain that so long as the air (or steam) pressure
is continued in the forked pipe the piston will automatically cut off
its supply and open its escape at each alternate end, and move back and
forth. Any boy can see how a backward and forward movement may be made
to give motion to a crank. All other details in an engine are questions
of convenience in construction, and not questions of principle or manner
of action.
Of older readers, I might request the supposition that, in Fig. 2, only
the valves A. and B. were automatically and invariably opened and closed
by the action of the piston-rod of Fig. 3, and that C. and D. were
controlled solely by the governor, before mentioned, which we will
suppose to be located at E. Then the escape of the steam ahead of the
piston must always come at the same time with reference to the stroke,
but the supply will depend upon the requirements of each individual
stroke, and the work it has to do, and afford to the piston a greater or
less push, as the emergencies of that particular instant may require.
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